Since harmful components such as hydrocarbon (THC), carbon monoxide (CO), and nitrogen oxide (NOx) are contained in exhaust gas of an internal-combustion engine of a motor vehicle or the like which uses gasoline as a fuel, it is necessary to simultaneously purify each harmful component using an oxidation-reduction reaction and exhaust the resulting constituent. For example, it is necessary to purify hydrocarbon (THC) by oxidizing the hydrocarbon (THC) to convert it to water and carbon dioxide, to purify carbon monoxide (CO) by oxidizing the carbon monoxide (CO) to convert it to carbon dioxide, and to purify nitrogen oxide (NOx) by reducing the nitrogen oxide (NOx) to convert it to nitrogen.
As a catalyst for treating exhaust gas from such an internal-combustion engine (hereinafter referred to as an “exhaust gas purifying catalyst”), three-way catalysts (TWC) which can redox CO, THC, and NOx are used.
Examples of such three-way catalysts which are known include a structure prepared by supporting a precious metal on refractory oxide porous particles such as alumina porous particles which have a high surface area and coating, with the precious metal-supporting porous particles, a base material, for example, a monolith-type base material made of a refractory ceramic or a metal honeycomb structure to form a catalyst layer.
However, the bonding strength between a precious metal as a catalytically active component and a base material is not so strong, and the specific surface area of the base material itself is not so large. Therefore, even if it is intended to allow a base material to directly support a precious metal, it is difficult to support a sufficient amount of precious metal in a highly dispersed state. Therefore, in order to allow a sufficient amount of catalytically active component to be supported in a highly dispersed state on the surface of a base material, a precious metal has been supported by a particulate catalyst carrier having high specific surface area.
Examples of this type of catalyst carrier which is known include porous particles made of a refractory inorganic oxide such as a silica, alumina, and titania compounds. Among them, activated alumina made of a mixture of gamma phase alumina and delta phase alumina has a particularly high surface area and is an excellent material as a catalyst carrier.
Incidentally, since exhaust gas of motor vehicles or the like has a high gas flow rate, the exhaust gas has posed a problem that the exhaust gas is hardly diffused to a deep part of a catalyst layer, preventing sufficient catalyst performance from being exhibited. Therefore, in order to increase gas diffusibility to improve purifying performance, the following proposals have been made in which voids are formed in a catalyst layer.
For example, Patent Literature 1 (Japanese Patent Laid-Open No. 2002-191988) and Patent Literature 2 (Japanese Patent Laid-Open No. 2002-253968) propose a NOx storage reduction type catalyst in which a precious metal and a NOx storing agent are supported by a coating layer made of a porous structure provided with pores each having a specific pore size to thereby increase the gas diffusibility of exhaust gas to improve the purification efficiency of NOx.
Patent Literature 3 (Japanese Patent Laid-Open No. 2004-025013) discloses an exhaust gas purifying catalyst comprising a honeycomb-shaped base material and a catalyst coating layer formed on the surface of the base material, in which the catalyst coating layer contains at least a powder of an oxygen storage release material, has pores each having a central pore size of 0.1 μm or more, wherein the pore volume of the pores in the range of a central pore size of ±50% is 0.05 cc/g or more.
Patent Literature 4 (Japanese Patent Laid-Open No. 2006-110485) discloses, as an exhaust gas catalyst to increase the gas diffusibility of exhaust gas in a catalyst layer to thereby improve catalyst efficiency, an exhaust gas purifying catalyst comprising at least a carrier and a plurality of layers formed on the carrier, wherein at least one layer in the plurality of layers contains a catalytic component; the layer has voids; and the voids have an average diameter of 0.2 to 500 μm.
Further, as a method for forming large voids in a catalyst layer, there is disclosed a method of forming large voids by adding carbon particles, resin particles, and the like and burning out the carbon particles and resin particles caused by calcining. For example, Patent Literature 5 (Japanese Patent Laid-Open No. 2012-240027) discloses a method of creating voids each having the same shape as that of a carbon compound material by mixing catalyst particles and the carbon compound material together with a solvent to prepare a catalyst slurry and burning out the slurry in a subsequent step.